Electrostatic writing



June 8, 1965 F. scHRbTER ELECTROSTATIC WRITING Filed Dec. 19. 1960 FIG.2.

FIG.3.

BY i

INVENTOR Fritz Schrfiter I I W I ATTORNEY United States Patent Claims. in. 346-44) The present invention relates to a device for electrostatically storing lines of charges representing images, particularly television programs, on strip-shaped carriers of any length, such as tapes, with the aid of a synchronously deflected electron beam, the intensity of which is modulated by the original image signal, which electron beam passes over or scans the carrier in a direction transverse to the direction in which the carrier moves, such that each complete transverse scan comprises the full width of one line of a picture. This will hereinafter be referred to as transverse writing.

It the transverse writing is to be accomplished electrostatically, the carrier strip must be a film of high insulating properties onto which the image charge applied by the electron beam can be made visible or otherwise rendered permanent to make it possible to reproduce the Written electrostatic image, for instance by dusting a fine opaque powder onto the charged film. The reproduction is particularly convenient if the powder is dusted on a transparent film so that ordinary projection by appropriate lighting can be used to make the stored image visible. This image can be projected upon a pickup tube the screen of which is scanned in a known manner to reproduce the image in electrical form. Alternatively, a light beam scanner can be used, as is customary in the television art for purposes of transmitting film.

Clear transparent films of practical commercially 0btainable materials, as, for example, Hostaphan, Tefion, or similar organic polymerization products, have the unfortunate characteristic that they will hold only a relatively small charge when subjected to electron bombardment by an electron beam for only a brief instant such as microsecond. These small charges are less than is desirable in order to obtain a sharp image of adequate contrast, or to obtain a satisfactory charge density for subsequent scanning. It has been found by tests that even if the intensity of the electron beam is reasonably high, the results of dusting are very spotty and the density of the dust is much less than would be expected on the basis of calculations. The light-dark contrast is insuiiicient. This is undoubtedly due, in part, to positive ions in the cathode ray tube, which ions are attracted by the negative charges at the surface of the carrier strip and tend to neutralize the same. However, other causes are also responsible for the fact that the charge density is inadequate.

It is, therefore, an object of the present invention to overcome the above disadvantage by providing for each image point such large charge densities that no harmful diminution of negative charges occurs, or that in the so-called Electrofax method, which is elaborated on below, there will be no loss of image points due to an insufficient amount of light thereat. This is accomplished (l) by storing or prolonging the charging time at each image point so that the impingement time covers a larger fraction of the total time interval corresponding with each line (the order of magnitude being second) and (2) by longitudinally advancing the carrier hand during each transverse writing scan by a distance equal to the width of one image point (it being assumed that an image point is square). As a result of the latter step, each transverse image line comprising a charge line remains 3,188,556 Patented June 8, 1965 exposed to the influence of the prolonged electron beam impingement (in the Electrofax process, the prolonged illumination peroid), throughout practically the entire total time interval of one line.

Additional objects and advantages of the present invention will become apparent upon consideration of the following description when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic sectional plan view of an apparatus according to the present invention.

FIGURE 2 is a fragmentary view of a modified apparatus similar to that of FIGURE 1.

FIGURE 3 is a fragmentary view of still another modified apparatus similar to that of FIGURE 1.

Referring now to the drawings, there is shown an electron beam tube 1 having a control electrode 2 to which the picture signal to be written is supplied by way of a lead 2a. A deflection system 3 serves to deflect the beam 4 in the direction of the picture lines which lie in the plane of the drawing. No means is employed for deflecting the beam in a plane normal to the direction of deflection ot' the system 3 and therefore the beam will always scan along the same path, this scanning taking place at the picture line frequency. The electron beam impinges on one side of a fine linear lattice 5 which serves to store the image-point charges. As is known in the art, the lattice 5 is coated on the side of the incoming beam with an insulation which can be positively charged to a greater or lesser extent by secondary emissions, this insulation being in the form of a layer vaporized onto the lattice. A linear equipotential cathode 6 is arranged parallel to the lattice 5 without being in the Way of the beam, and this cathode 6 emits a strong drift of low-energy electrons if the potential of this cathode relative to the lattice 5 is appropriately selected. The cathode 6 is of tubular shape and is heated indirectly by a bifilar heating coil 7 which, for the sake of simplicity, is shown in the drawing as a straight line. The current through the coil 7 is taken from points 8, 8a. The reaction for making the heater for the cathode 6 in the shape of a bifi lar coil is to prevent the generation of an interfering magnetic field.

As is known, the passage of the low-energy electrons through the interstices of the storage lattice 5 places charges along the lattice in a straight line according to the charge distribution modulated on the electron beam 4. This distribution is also reproduced in compressed proportions in the space at the right of the lattice 5, and a magnetic electron lens 1G is provided to focus this reproduction in the plane of a slot 11, as will be explained below. The drawing omits, for the sake of simplicity, the rotation imparted to the electron beam; in actual practice, however, due allowance for this can easily be made. A voltage potential V produces a very homogeneous electric field which accelerates the electrons passed by the lattice 5. The voltage V is shown schematically only, but in practice conductive rings or a spiral 9 of low pitch, made of a suitable material such as graphite, can be applied to the inner surface of the tube, in a manner known in the art in conventional cathode ray tubes. As stated above, there will appear in the plane of the slot '11 a linear image corresponding with the charge distribution on the lattice 6. This image is in the form of point-shaped relatively high-energy electron concentrations of different intensities.

This energy can be used in various ways to obtain a permanent record of the image to be recorded. One of these ways is that the electron concentrations are used directly for the generation of light, in which case a light screen 15 (FIGURE 2) is placed in the plane of the slot 11; or the electrons are allowed to passed out of the tube through a narrow slot-shaped Lenard-type window 14 (PEGURE 1). In the former, the Electrofax method can be used, wherein a pre-charged Electrofax band 17 is passed in closest possible optical contact along the very thin transparent window 15 of the tube, which window has a luminous phosphorous layer on the inside, so that the zinc oxide layer of the Electrofax paper 17 is discharged at the illuminated portions. In order to accomplish this, the Electrofax band v17 is precharged by an electrode 16 (FIGURE 2) and is then run over a roller 13 which is as nearly as possible tangent to the plane of the image line. The charge distribution remaining on the Electrofax paper '17 can then be made visible by dusting, and thereafter the image can be fixed by heating.

Another way of utilizing the electrons coming through the Lenard-type window 14 F-IGURE 1) is to catch them on a strip of insulating material, which is designated by the reference numeral 12, and which can also run over the roller 13, whereafter the strip is dusted so as permanently to store the charge pattern.

In any case, the present invention resides basically in applying at each image point on the record carrier strip the amount of energy or charge quantity necessary to obtain a sufiicient charge on or discharge from the carrier by electronically storing an entire image line on a lattice throughout a large fraction of the time assigned to such line and continuously focusing a corresponding compressed line on the carrier during said large fraction of time.

If the electron-optical components of the apparatus are properly constructed, the arrangement shown in the drawings makes it possible to obtain a very great image sharpness inasmuch as the image line can be expanded to a relatively long length on the linear storage lattice 5. For example, if the line is resolved into 600 image points and if the line is 12 cm. long, a width of 0.2 mm. is available for each image point. This sharpness is proportionately preserved upon the reduction in size at the slot 1-1. This reduction in size is necessary in order to make it possible to store long picture sequences in a narrow and compact carrier strip. It will be appreciated that electron-optical systems capable of very substantially reducing relatively large primary emission surfaces are already known in X-ray image amplifiers. In the instant case, the problem is simplified inasmuch as only one emission line at a time is involved. The electron-optical system shown in the drawings can be modified in various ways. For example, the reduced emission representation of the low-energy electrons passing through the storage lattice 5 and entering the acceleration chamber can be carried out by a purely electrostatic system, as is customary in certain X-ray picture amplifiers. In this case, the linear storage lattice must be spherically curved in the line deflection plane, i.e., the lattice must be in the form of a narrow strip-shaped cutout taken from a spherical surface substantially along a great circle thereof. An after-focusing system must then be used so that the electron beam will write with equal sharpness along the arc of the line. This is done by providing a correcting voltage of picture line frequency which varies symmetrically about the axis in such a manner that the same full sharpness of the writing beam will prevail along the length of the storage lattice This method and the means for carrying it out are known in the art.

In order to reduce the transfer time, in the abovedescribed arrangement with an insulation-covered control lattice, to sufficiently small values, it may be necessary to provide an auxiliary electron beam in the writing chamber (the left portion of FIGURE 1) which, during the return-trace of the beam obliterates the charges present along the storage lattice sufliciently rapidly, and restores at all .points the same operating potential prior to writing the next image l-ine.

It is also possible to pass the carrier strip directly through the reproduction chamber of the tube 1 by passing the strip through pressure lock-s; or to run the entire carrier within the interior of the tube (see FIGURE 3). In this case, a carrier such as strip 12 is run past a slot 18, the

' entire carrier being located within an evacuated portion 1a of the envelope l, which portion 111 can be easily removed there-from. Thus, an ample charge density will be produced on the carrier. For practical purposes, however, it is desirable to have the carrier run exterio-rly of the tube. On the other hand, if the process is carried out in a vacuum, the electron energy can be more fully utilized. Also, it is then possible to reduce the amount of electrons, and, furthermore, charge leakage into the air from the carrier is avoided.

If the apparatus disclosed herein is operated according to the Electrofax process, in which a light screen is arranged in the plane of the slot 11 and seals the tube at this place, then a very narrowstrip-shaped screen, which covers the slot-shaped opening, will be sufiicient, thanks to the excitation of the luminous material which occurs always along the same line. It is therefore possible to make the transparent screen carrier extremely thin without this screen being blown inwardly by the outer air pressure.

The arrangement thus makes it possible, by appropriate fashioning of the edges of the slot, to make use of a large aperture angle of the light beam emitted by the screen, assuming the carrier, which consists of Electrofax paper or equivalent material and which carries an appropriate pre-charge, is passed very closely, i.e., virtually tangent, to the luminous window screen. In this way, a very high optical efficiency is obtained.

If a fine-mesh modulation grid is arranged close to the lattice 5 on the side at which the beam 4 impinges on the lattice 5, and if this modulation grid has applied to it the image signal voltage while the strength of the beam 4 is maintained constant, every image element of the storage line is transferred so rapidly that all of the lines can be written on the same scanning path, without this resulting in blurring or in the formation of afterpresent invention is susceptibleto various modifications,

changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

I claim:

1. Apparatus for electrostatically providing a charge distribution on a carrier of insulating material in accordance with input signals preparatory to fixing of the image by dusting of a powder on the carrier, comprising, in combination: cathode ray beam generating means; beam deflection means for repeatedly sweeping said beam in a transverse direction; beam modulation means for modulating the intensity of said beam in accordance with said input signals; a mechanical storage lattice disposed in said transverse direction and impinged upon by said beam when swept; a source of low-energy electrons on one side of said lattice and extending therealong in the direction of the beam impingement; low-energy electron accelerating means on the other side of said lattice; a sealed evacuated envelope surrounding said beam generating means, said beam modulating means, said mechanical storage lattice, said electron source and said acceleratfocusing said accelerated low-energy electrons of the carrier to provide a reproduction of said density distribution thereon and means for longitudinally advancing the carrier during each transverse writing scan by a distance equal to the width of one image point, whereby each beam line pertaining to a charge line is temporarily stored in said mechanical storage lattice and each transverse image line comprising a charge line is exposed to the influence of a prolonged electron beam impingement throughout substantially the entire total time interval of one line.

2. In apparatus as set forth in claim 1, a Window in the envelope and disposed in the path between said lowenergy electron source and the carrier, said advancing means advancing said carrier past said window in close proximity therewith.

3. In apparatus as set forth in claim 2, said window comprising a thin Lenard-type window located in the path of the low-energy electrons and adapted to pass the latter to impinge upon the carrier to provide a concentrated distribution of electrons thereon.

4. In apparatus as set forth in claim 2, said window comprising a luminous screen disposed in close proximity to the carrier and receiving said focused low-energy electrons and glowing in proportion to the bombardment thereby, and said carrier comprising a web coated with a photo-conductive material; and material pre-charging means applying to the material a uniform charge distribution to be drained off in proportion to the exposure to light from said screen.

5. In apparatus as set forth in claim 1, said advancing means being arranged for advancing said carrier within said envelope for direct impingement thereon by said focused low-energy electrons.

References Cited by the Examiner UNITED STATES PATENTS 2,212,249 8/40 Schroter 346-74 2,267,251 12/41 Okolicsanyi 346-74 2,716,048 8/55 Young 346-74 2,736,770 2/56 McNaney 346-74 X IRVING L. SRAGOW, Primary Examiner. NEWTON N. LOVEWELL, Examiner. 

1. APPARATUS FOR ELECTROSATICALLY PROVIDING A CHARGE DISTRIBUTION ON A CARRIER OF INSULATING MATERIAL IN ACCORDANCE WITH INPUT SIGNALS PREPARATORY TO FIXING OF THE IMAGE BY DUSTING OF A POWDER ON THE CARRIER, COMPRISING, IN COMBINATION: CATHODE RAY BEAM GENERATING MEANS; BEAM DEFLECTION MEANS FOR REPEATEDLY SWEEPING SAID BEAM IN A TRANSVERSE DIRECTION; BEAM MODULATION MEANS FOR MODULATING THE INTENSITY OF SAID BEAM IN ACCORDANCE WITH SAID INPUT SIGNALS; A MECHANICAL STORAGE LATTICE DISPOSED IN SAID TRANSVERSE DIRECTION AND IMPINGED UPON BY SAID BEAM WHEN SWEPT; A SOURCE OF LOW-ENERGY ELECTRONS ON ONE SIDE OF SAID LATTICE AND EXTENDING THEREALONG IN THE DIRECTION OF THE BEAM IMPINGEMENT; LOW-ENERGY ELECTRON ACCELERATING MEANS ON THE OTHER SIDE OF SAID LATTICE; A SEALED EVACUATED ENVELOPE SURROUNDING SAID BEAM GENERATING MEANS, SAID BEAM MODULATING MEANS, SAID MECHHANICAL STORAGE LATTICE, SAID ELECTRON SOURCE AND SAID ACCELERATING MEANS; ELECTRON LENS MEANS IN THE PATH OF SAID ELECTRONS AND DISPOSED BETWEEN THE ACCELERATING MEANS AND THE CARRIER, THE MECHANICAL STORAGE LATTICE PERMITTING THE LOW-ENERGY ELECTRONS FROM THE SOURCE TO PASS TOWARD THE ACCELERATING MEANS WITH A DENSITY DISTRIBUTION IN THE 